Radial outgoing feeder overcurrent protection
The basic requirements for feeder overcurrent protection are adequate sensitivity
and operation speed taking into account the minimum and maximum fault current
levels along the protected line, selectivity requirements, inrush currents and the
thermal and mechanical withstand of the lines to be protected.
Often the above requirements can be best fulfilled using multiple-stage overcurrent
units.
Figure 74 shows an example of this. A brief coordination study has been
carried out between the incoming and outgoing feeders.
The protection scheme is implemented with three-stage numerical overcurrent
protection where the low-set stage PH3LPTOC operates in the IDMT-mode and
the two higher stages, PH3HPTOC and PH3IPTOC, in the DT-mode. Also the
thermal withstand of the line types along the feeder and the maximum expected
inrush currents of the feeders are shown. Faults occurring near the station where
the fault current levels are the highest are cleared rapidly by the instantaneous stage
to minimize the effects of severe short circuit faults. The influence of the inrush
current is taken into consideration by connecting the inrush current detector to the
start value-multiplying input of the instantaneous stage. This way, the start value is
multiplied with a predefined setting during the inrush situation, and nuisance
tripping can be avoided.
1YHT530004D05 D Section 4
Protection functions
615 series 169
Technical Manual
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